In the semiconductor manufacturing industry, high-temperature furnaces are used to create semiconductor junctions and circuits by exposing semiconductor materials to chemical gases under high temperature. To regulate the desired chemical reactions, and to ensure consistent quality, accurate control of furnace temperature is required The furnace temperature may exceed 1000 degrees Centigrade.
Control may be achieved by creating a feedback loop in which a furnace temperature regulator is coupled to a temperature sensor in the furnace. The sensor detects internal furnace temperature and produces an output electrical signal proportional to the temperature. The signal is coupled to a monitor computer, or to a temperature display, enabling either manual or automatic adjustment of the furnace.
Conventionally, such temperature sensors comprise a precious metal thermocouple having a junction of two different precious metals or alloys. Heat applied to the two metals causes a thermo-electric potential difference to develop between the metals, which produces an output voltage signal. Precious metals used in conventional thermocouples may include platinum-rhodium alloys which are intrinsically temperature resistant, but which have numerous disadvantages including extremely low output voltage, susceptibility to electromagnetic interference, low signal-to-noise ratio, and slow response time.
To overcome these disadvantages, optical fiber thermometers (OFTs) have been developed. An optical fiber thermometer system may comprise a blackbody radiator on an optical fiber having a "hot" end in a furnace under test and a "cold" end coupled to receiving and decoding electronics. The blackbody radiator may be precious metal secured to the hot end of the fiber. The cold end of the fiber is coupled, outside the furnace under measurement, to a photodiode receiver assembly which includes amplification and temperature conversion electronics. Several fibers can be bundled to increase signal strength or to enable temperature sensing at several distributed locations in the furnace.
Most conventional optical fibers, such as those used in telephony and computer data transmission, cannot withstand the high temperatures of semiconductor furnaces. Therefore, some known devices employ sapphire fibers or rods, which are highly heat-resistant, which provide good optical qualities, and which enable easy affixation of a precious metal blackbody radiator. For example, U.S. Pat. Nos. 4,576,486, 4,750,139, and 4,845,647 disclose OFT systems using a high temperature sapphire fiber in a furnace coupled to a low-temperature silica fiber outside the furnace. In these patents a platinum blackbody is formed on a sapphire fiber by sputtering a coating of platinum on the fiber. However, this method is undesirable because the sputtered coating may be too thin for proper sensing. The coating or blackbody must be at least as thick as the longest wavelength of radiation to be sensed. Sputtering may produce a blackbody which is thinner than this wavelength.
U.S. Pat. No. 3,626,758 (Stewart et al.) shows a sapphire fiber for high-temperature sensing having a metallic coating sputtered on the tip which serves as a blackbody radiator.
Platinum blackbody radiators are desirable, but depositing a hot molten platinum blackbody radiator on a silica fiber will melt the fiber. Causing adhesion of a precious metal blackbody radiator on a silica fiber is difficult.
An article by J. P. Dakin and D. A. Kahn, "A novel fibre-optic temperature probe," Optical and Quantum Electronics 9 (1977), p. 540, proposes use of a single silica fiber having a maximum operating temperature of 1,100.degree. C. for direct temperature measurement. The fiber is encased in a fine stainless steel tube and terminates in an opaque end cap comprising thin metal film, the exact composition of which is not disclosed. The reference fails to teach how to fire a platinum blackbody onto a silica fibre.
U.S. Pat. No. 4,794,619 (Tregay) shows a thermally emissive radiator cavity drilled into the end of a low-temperature fiber which may comprise glass. The cavity may be coated with an enhancing compound such as an oxide of aluminum, silicon, zirconium, or yttrium, as stated in column 5, line 17. The cavity is not a true blackbody.
U.K. Patent 2,045,921 (Dakin/Plessey) shows a temperature measuring probe with an opaque cap of thin metal acting as a black body fitted to the end of an optical fiber which is connected to a radiation detector. However, this patent does not disclose how the cap is retained on the end of the fiber. Adhesives could be used but could interfere with energy transmission.
Therefore, those who use OFT sensors desire to have a precious metal blackbody radiator on a sapphire or silica fiber for optical temperature transmission.